The instrument is one of 10 that will help the rover in its mission to determine the planet's habitability — whether it ever was or still is an environment that could support microbial life such as bacteria.

It was designed by University of Guelph physics professor Ralf Gellert and built by Richmond, B.C.-based MacDonald, Dettwiler and Associates Ltd.

How an APXS works

Gellert was part of the German team that designed the previous Mars rover APXS instruments. He is now a physics professor at the University of Guelph. ((Courtesy University of Guelph Physics Department))

An alpha particle X-ray spectrometer, or APXS, contains a radioactive source that shoots alpha particles at a rock sample. When the particles hit the nuclei of atoms inside the rock, several things can happen. The alpha particles can:

Bounce off, losing some of their energy in the process. The amount of energy they lose depends on the mass of the nucleus they hit, which is different for different elements. Scientists can figure out which element they hit by comparing the energies of the alpha particles before and after they bounce off the rock.

Be absorbed by certain elements. That will cause the release of a smaller particle called a proton from the nucleus of the atom, and the proton's energy will depend on the type of element that released it.

Eject electrons, which orbit the nucleus of the atom in concentric shells, or layers, of various energy strengths, from an inner shell. Electrons from the outer shell will fall down to replace them. In the process, they release energy in the form of X-rays. The amount of energy depends on the type of element.

Detectors in the spectrometer measure the energy of alpha particles, protons and X-rays coming off the sample, allowing the machine to determine what elements are present and in what amounts.

A less-advanced version of the instrument was carried aboard the Mars Exploration rovers Opportunity and Spirit, which landed on Mars in 2004. Those devices, built by a German team that included Gellert, helped analyze a salt lake and other hints of water.

APXS machines in laboratories on Earth are used by geologists to figure out what elements are present in a certain rock and how much of each element there is, Gellert said. Such instruments would typically weigh about a tonne and consume a couple of kilowatts of electric power — qualities that are problematic on a space mission, where every ounce counts and access to electrical power is limited.

For Mars, Gellert designed an APXS with electronics that are roughly the size of a trade paperback book and weigh just 500 grams. They will be nestled in the rover's belly and connected to a sensor on the rover's arm that is roughly the size of a pop can. The entire instrument uses just two watts of power — 20 times less than a 40 watt light bulb.

"For sure, it's not as good as a terrestrial instrument," Gellert admitted.

But it's able to face challenges that no APXS designed for Earth could withstand.

It can survive the massive shaking of a rocket launch and the thermal stress — the expanding and contracting of different parts — caused by huge temperature fluctuations that range from -80 C at night to -20 C during the day.

The machine must be extremely robust because there is no way to repair it from Earth — which is more than 54 million kilometres from Mars.

Despite its hardy design, the Mars APXS wouldn't work on Earth except in extremely controlled laboratory conditions, Gellert said, because it can only operate in a vacuum or near-vacuum and at very cold temperatures.

"You get this vacuum on the moon or Mars … for free," Gellert said.

Because the device requires a radioactive source, there are lots of safety requirements on Earth that the designers of a Mars version don't need to worry about.

"No one actually really cares what you do with radioactive salts on a different planet," said Gellert.

The new Mars APXS machine is faster and more versatile than its predecessors. While the earlier version needed all night to take a single measurement, the new one only needs a couple of hours.

Daytime measurements possible

And while the earlier versions could only operate at temperatures below -40 C — making them suitable for night measurements only — the new model can operate at daytime highs of -20 C.

The APXS data will be used to guide measurements from other instruments such as cameras, radiation detectors and environmental sensors and interpret data measuring other qualities of the same Martian features.

"Everyone contributes a piece of the puzzle, and then you put it together," said Gellert.

He and his students helped analyze the data that came back from the previous Mars missions.

The new instrument has already survived vibration tests, in which it is strapped to "something like a big loudspeaker," and thermal tests in a giant freezer, where the temperature is raised and lowered repeatedly.

It was delivered to NASA in December 2008 for a 2009 launch, but that mission was delayed until 2011.

Prior to the launch, Gellert will have to assemble and install the radioactive sources, which can't be pre-installed too far ahead of time because they decay.

But at the moment, the device is resting quietly ahead of its long, long journey.